This Application claims priority to UK Application No. 0010184.0, Filed Apr. 26, 2000.
1. Field of the Invention
This invention relates to a prosthetic foot.
2. Background Art
A very large number and variety of constructions of prosthetic feet are known. It is desirable to have a prosthetic foot that is reliable, lightweight, and that provides the flexibility and response matching as closely as possible to that of the natural foot it is to replace. It will be appreciated that different stresses and requirements arise depending on the age, weight, amputation level and general agility of the amputee.
U.S. Pat. No. 5,037,444 discloses a prosthetic foot that has a forefoot portion with a heel portion demountably and interchangeably connected thereto. The forefoot and heel portions are fabricated from polymer impregnated and encapsulated laminates, such as carbon fibres and/or fibreglass or synthetic fibres such as Kevlar. The demountable connection of the heel portion permits interchangeability of heel and forefoot portions to match the weight, stride and activity schedule of the wearer of the foot. The foot and heel portions contain spring sections, and are rigidly secured together.
U.S. Pat. No. 4,547,913 discloses another simple construction of composite prosthetic foot and leg having leg, foot and heel portions of substantial elastic flexibility. The three portions are rigidly joined together within an encircling binding.
WO-A-96/04869 discloses a more complicated foot prosthesis having a cushioned ankle, in which an ankle block, formed of a resilient material or a bladder having desired compliance and energy return characteristics, is sandwiched between a lower foot plate element that extends from toe to heel and an upper ankle plate element. Spring elements may be embedded in the ankle block or bladder to increase the rigidity of the prosthetic foot and to improve the degree of energy storage and return.
It is one object of the present invention to provide a prosthetic foot that is lightweight, of relatively simple construction, and which offers a natural feel that can be adjusted to the requirements of the wearer.
In accordance with a first aspect of the present invention, there is provided a prosthetic foot comprising:
a substantially two-dimensional, preferably elongate, resilient toe member;
a substantially two-dimensional, preferably elongate, resilient heel member; and
a substantially rigid carrier member; wherein one end of each of the toe and heel members is connected to the carrier independently of the other of said members.
The foot of the invention can thus provide two spring elements, for the toe and for the heel, which are independently secured to a rigid component. By this means each element can be individually selected to provide the wearer with the required characteristics and responses at heel-strike, foot-flat, and toe-off independently of the other element. Thus, a pair of elements can be matched to each other to suit a specific individual. Furthermore, the resilience of one or both of the elements of a foot may be adjustable, either by the wearer on demand, for example to accommodate different levels of activity of the user, or by being pre-set during initial fitment of the prosthesis to the wearer.
It is important to match the heel and toe members in order to achieve a smooth transition from heel strike to toe off with maximum energy storage and return. If the foot is not balanced, the gait can become jerky and less energy efficient. Some of the effects of poorly matched functional areas in a foot are listed below:
1. Heel too soft: Sinking at heel strike. Toe appears hard (see below).
2. Heel too hard: Jarring at heel strike. Rapid progression to toe off.
3. Toe too soft: Spring does not return sufficient energy. Rapid progression through roll over to toe off.
4. Toe too hard: Difficult to progress to toe off. Back pressure on stump.
5. Heel and toe too hard: Little energy absorbed or returned. Jerky, jarring action.
6. Heel and toe to soft: Foot does not return energy. Amputee spends extended time on foot during support phase.
Since in the foot of the invention, the resilient members will be supplied as a matched pair, the alignment shift feature will them be used to fine tune the foot rapidly to the amputee""s requirements.
Preferably, the toe member of the foot extends forwardly of the foot from its connection with the carrier member and the heel member extends rearwardly of the foot from its connection with the carrier member, and the toe and heel members overlap one another in the region between the connections. The overlapping relationship of the toe and heel members allows each one, acting as a spring element, to be individually designed to the required specification, including specifically allowing for its length to be optimised, whilst maintaining the overall length of the completed prosthesis within the natural shoe size of the wearer. Preferably the toe member extends over the heel member. The arrangement in which the toe member lies above the heel member allows the use of bigger springs, whilst minimising the overall height of the foot. This has the advantage that the foot of the invention can be fitted even to amputees having relatively long residual stumps. Furthermore, the longer the resilient members can be, the straighter they can be made for any given required wearer characteristics. A straighter member not only is easier to manufacture, but problems of high stress resulting from significant curvature are also minimised. Since a resilient foot member may be of laminate construction, and act structurally as a beam, it could become loaded in a direction which tends to open the radius, resulting in failure by delamination. This is overcome, or at least alleviated, either by enclosing the member within an encapsulating layer (which can be comparatively expensive) or by thickening the member locally (which can reduce the flexibility of the beam). Such problems can thus be avoided, or at least minimised, by the straighter members that are allowed by the foot construction of the present invention.
It is to be appreciated, however, that with suitable materials, it may be possible to design the prosthetic foot with toe and heel members that do not need to overlap. In such a foot, the rigid carrier member will be located longitudinally intermediate the toe and heel members.
The carrier member may define an enclosure, preferably a rectilinear cage, wherein one end of one of the toe and heel members extends into the enclosure and is secured therewithin, and the other of the toe and heel members is secured to the outside of the enclosure. The preferably cage-like structure of the carrier member can thus provide the required rigidity for the independent mounting of the two resilient foot members, whilst keeping the weight to a minimum. The rigid carrier also serves as the attachment point of the foot to the limb, thus avoiding the need for any stiffening that would otherwise be necessary, and disadvantageous, if the ankle of the limb were to be connected directly to one of the resilient foot members.
In accordance with a second aspect of the present invention, there is provided a prosthetic foot comprising a resilient, and preferably elongate, toe and/or heel member, in which the or each member is of generally two-dimensional configuration but is of channel-section, for example U-, V-, or C-shaped section.
Such a configuration of member can thus not only allow deflection in the Medial-Lateral (M-L) direction whilst maintaining relatively rigidity in the Anterior-Posterior (A-P) direction, but can do this using a relatively thin-walled, and thus, lightweight, component. In general, a thin-walled channel section beam has the following advantages over a rectangular section beam:
(a) it can carry bending loads more efficiently, and consequently it uses less material and is therefore lighter, and potentially cheaper;
(b) it has comparatively low torsional stiffness, which allows the toe member of a prosthetic foot, for example, to twist and to conform to uneven surfaces. Conventional prosthetic foot configurations achieve M-L compliance through the use of either a split toe member, which is potentially noisy and structurally weaker than a non-split configuration, or a separate resilient ankle, which adds weight and is also to some extent, and disadvantageously, energy absorbing; and
(c) it is better suited to the strength characteristics of a composite material. Because of the fibre buckling failure mode of a composite material under compressive load, typical materials are generally stronger in tension than in compression. In a rectangular beam section under bending, this comparatively low compressive strength becomes the limiting stress for the beam. A channel beam section, however, has an offset neutral axis in bending and this allows the high tensile strength to be better exploited and thus the efficiency of the overall beam design to be improved.
It will be appreciated that a toe and/or heel member of the said first aspect of the prosthetic foot of the invention may be formed in accordance with the said second aspect of the invention.
The toe and heel members may be releasably mounted to the carrier member, for example using bolts and nuts, or alternating they may be permanently secured thereto following final fitting trials with the amputee. Thus, foot members having different characteristics, for example spring rate, may be provided for different activities of the wearer, for example, normal walking or a sporting activity. It is also envisaged that a foot having given toe and heel members may be adjustable for different activities. For example, an adjustable stop may be provided, preferably mounted on the carrier member, to limit the flexing of the toe and/or heel member with respect to the carrier member.
The toe and heel members may be made from any suitable material, but preferably from high-strength, lightweight, resin-impregnated fibrous material. Carbon fibre, glass fibre, or Kelvar composites, for example, are particularly suitable materials. The carrier member is also preferably formed from a lightweight, high strength material, and may be formed as a moulded composite material. It is also envisaged that the carrier member may be made from a low density metal, such as aluminum or titanium, or it may be made from stainless steel. The toe member may be slit longitudinally along at least part of its length, to facilitate lateral movement of the foot in use.
The toe, heel and carrier members will usually be enclosed within a cosmesis, with the ends of the toe and heel members that are remote from their attachment to the carrier member being secured thereto, preferably being retained within reinforced slots of the cosmesis.
Since the foot of the invention may be assembled from the three components discretely, it is possible to make the final assembly within the shell (cosmesis) itself. This allows a relatively high level of interlock, without the stretching of the foot shell that can occur with conventional configurations. A secure fitment of the toe and heel members within the foot shell is required in order to avoid the generation of noise in use.
In a third aspect of the present invention, there is provided a prosthetic foot in accordance with one or both of said first and second aspects of the invention, wherein a further resilient foot member is secured to the said toe and heel members at locations spaced apart from their securement to the carrier member.
Preferably, the further foot member, which preferably is substantially two-dimensional, is secured at one of its ends to the heel member at its end remote from the carrier member. The further foot member may be secured at its other end to the toe member at a region thereof intermediate its attachment to the carrier member and its free end.
The toe, heel and further foot members are preferably of generally elongate S-shape.
In accordance with a fourth aspect of the present invention, there is provided a method of forming a prosthetic foot, wherein the foot comprises a resilient toe member, a resilient heel member, a substantially rigid carrier member, and a cosmesis, in which the three said members are secured together and to the cosmesis subsequent to the introduction of the three members into the cosmesis.